Key Uncertainties in Projections of Future Climate

A great deal is known about past, present, and projected future climate change, especially at large (continental to global) scales. For example, there is high confidence that global temperatures will continue to rise, that the rate and magnitude of future temperature change depends strongly on current and future rates of GHG emissions, and that climate change—in interaction with other global and regional environmental changes—poses significant risks for a number of human and natural systems. Global climate models and, increasingly, regional techniques are also starting to provide useful information about future climate and climate-related changes on local to regional scales. Some of these projections—such as increases in extreme heat events and Arctic sea ice—are quite robust, while others are somewhat more speculative.

There are, however, several aspects of future climate change that remain more uncertain, and these represent some of the most important and active areas of current scientific research (see Research Needs at the end of this chapter). The uncertainties in climate projections can be categorized into two main sources: (1) uncertainties in future climate forcing and (2) uncertainties in how the climate system will respond to forcing, which includes both the known limitations of global climate models (such as an inability to resolve individual clouds) and the fact that the climate system is complex and might exhibit novel or unanticipated behavior in response to ongoing climate change.

The first of these categories, uncertainties in future climate forcing, was discussed in the Future Climate Scenarios section earlier in the chapter. The spread among the three colored curves in Figure 6.20 provides a rough indication of the importance of this uncertainty in terms of the magnitude of future climate change. As discussed above, and described in further detail in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c), future climate forcing depends strongly on the choices that current and future human societies make, especially regarding energy production and use. However, actions that might be taken to limit the magnitude of future climate change, or adapt to its impacts, have not yet been fully and systematically integrated into climate forcing scenarios and evaluated across a range of different climate models to determine how they might ultimately affect both climate and other aspects of the Earth system.

As an illustration of some of the uncertainties present in climate model projections, Figure 6.22 shows projections of temperature change over North America from 21 different models, each using the same scenario of future climate forcing. Several robust features emerge from these projections—for example, all of the models project a

FIGURE 6.22 Projected warming for the 21st century (difference between 2080-2099 temperature and 1980-1999 temperature) for the North American region using 21 different climate models, all using the same scenario of future GHG emissions. The mean (average) of the 21 model experiments is also shown in the bottom right panel. Several robust features are eviden t, including enhanced warming over land areas and higher latitudes. Differences among the 21 projections are indicative of some of the uncertainties associated with model projections. SOURCE: Christensen et al. (2007).

FIGURE 6.22 Projected warming for the 21st century (difference between 2080-2099 temperature and 1980-1999 temperature) for the North American region using 21 different climate models, all using the same scenario of future GHG emissions. The mean (average) of the 21 model experiments is also shown in the bottom right panel. Several robust features are eviden t, including enhanced warming over land areas and higher latitudes. Differences among the 21 projections are indicative of some of the uncertainties associated with model projections. SOURCE: Christensen et al. (2007).

substantial overall temperature increase, with stronger warming over land areas and at higher latitudes. Most of the models show somewhat less warming over the southeastern United States and a slight cooling, or at least less warming, over the western North Atlantic Ocean south of Greenland. In other regions, however, the exact pattern and magnitude of projected warming varies considerably among models. Typically, the average of many climate model simulations represents a more robust projection than any individual projection (Randall et al., 2007), so the average of these model calculations (shown in the bottom right panel) can be thought of as the most reliable prediction of future temperature change over North America. Differences among models indicate some (but not all) of the uncertainty in this "multimodel mean" projection. Analyses of the differences among models—such as CMIP3 and previous model intercomparison projects—are also a key tool for model development.

The other main type of "known" uncertainty in model-based projections of future climate change is associated with processes that are either not resolved or not very well simulated in the current generation of global climate models. These processes, which are discussed in further detail in the Research Needs section at the end of this chapter, include clouds and aerosols, the carbon cycle, ocean mixing processes, ice sheet dynamics, ecosystem processes, land use-related changes, and extreme weather events such as hurricanes, tornadoes, and droughts. Another key research area is the relationship between regional modes of variability and global climate change, including the possibility that regional variability modes may shift in response to either regional or global human activities.